Unless blackouts or brownouts are going to be allowed to increase in frequency from very rare at present, sources of immediate dispatchable electricity are needed. Large scale hydro plants, battery storage, are expensive to have sitting around. Coal can take several hours to ramp up generation , Gas can be much faster, a few hours from a cold start.

Nighttime is predictable. I'm specifically include bad weather in solar / wind generation which can mean zero output. Can be for days. Storage needs to be reserved for these events.